nonono! The other way around!
It sustains only D: Conc spells (as per Maintaining the Demanding Spell).
The Ring certainly does not need concentration to be maintaned!
Sorry for not making things clear.
Where do you think you are getting this?! 
The Guidelines clearly say "Sustain a spell", not "Sustain a Spell that has D:Conc".
Any Duration can be sustained in this manner. (Whether or not "any" includes "Momentary" is up to the SG/Troupe - always room for interpretation and Houserules.)
Maintain the Demanding Spell chose that one target for that one example - nothing in the Guidelines says a similar effect couldn't sustain other Durations.
The Guidelines define the Spells, not the other way around.
I'm not "getting this" anywhere really.
I'm trying to explain what I meant above, and what I'm fairly certain others meant.
I'm also pointing out the only example we have of this guideline being used.
I personally believe that case is what was meant, but agree that it is not what was written.
For other changes of duration, I would personally refer my fellow players (and now you) to the MuVi guidelines.
This may or may not be house ruling, but I find little solid argument that any other interpretation is more valid.
Ah - it certainly sounded like you were referring to the way the rules work, not was merely suggested as one interpretation.
Why look to MuVi for what is clearly a ReVi effect?
That's ok too. I was just wondering.
My apologies, I was merely confirming what I thought (and still think?) another poster had stated.
But that is not clear.
Would you conceed there is a difference between sustaining a spell, and extending it?
The only spells that need to be sustained are those of duration Concentration.
And should you wish to extend the duration of a spell, you should refer to MuVi, as you are changing a parameter of said spell, exactly as per Wizard's Reach, save only that you are affecting duration rather than range.
On this we agree fully.
Hm, that's a really good point.
Tellus' point seems valid to me. And we interpreted the rules in the same way from the beginning. But each his own way.
Our group has gone farther: we banned all "sustaining" ReVi spells. We felt "sustaining" is merely another word for extending the duration of a spell, and this should be MuVi. In fact, when we began playing 10 years ago, most mage had his version of sustaining the demanding spell (often with duration: sun), using it on our spontaneous spells with great effect...too great effect, in fact.
The initial spell of this thread, Circle of the Demanding Spell Sustained, seems perfectly fine to me btr. I like the idea of targeting many spells with a Target:circle, haven't tought of that.
Hmmm... that last is an assumption, but not an unreasonable one. The RAW don't specify such a distinction, but they could be read as inviting it. Hrmmmm... indeed... 
I'm not sure I accept it, but I can't say that I dislike it.
I'm not sure that both would not work - the only (apparent?) diff is in magnitude, but nothing btr (that I can find) clearly states that one or the other is more/less in/appropriate.
While this sounds reasonable and almost convinced me, it does run contrary to the way the term "sustain" is (repeatedly) used in the Spell Binding mystery (TMRE) as a synonym of "extend" [a spell beyond its normal Duration].
Different author.
(Yeah, a word as used in a later work does not define a word as used in a previous work - unless that word is specifically being (more clearly) defined as a Game Term. This ain't that.) 
It does not really matter. It's still Ars Magica canon, and it ruins the argument about sustaining being different from extending.
No, ez - it's not. There is nothing in canon that states "Every use of 'sustain' is synonymous with 'extend' ", nor anything remotely close. Much less a statement referring to [i]previous[/u] uses in other books. An interpretation of that meaning within that one book is just that - an interpretation. Sorry, but - no.
Let me make myself clear.
There is canonical evidence that "sustain" is used as a synonym of "extend".
There is no canonical evidence that the two are ever used with a different meaning, since the only point where they may be is indeed the controversial one we are discussing.
By Occam's razor, all else being equal, it's just simpler to assume that the same word means the same thing throughout the game line.
Unfortunatly, they are being used synonimously by different authors, who may very well not have been aware/noticed the slight difference smply becaue it is only relevant in this specific context. At most, this proves nothing and leaves us back at square one, that we know nothing.
Unfortunatly Occam's razor is rather invalid, being an argument rutinely disproven in high schools.
Points for being in the medieval paradigm though.
It's not clear what you mean with "at most".
It certainly proves that we cannot assume that "extend" is different from "sustain".
It suggests that we can assume the opposite, since it's more reasonable to assume that (even different) authors are consistent than inconsistent, all else being equal.
Actually, Occam's razor is one of the foundations of modern scientific thought, and is constantly used throughout science. Of two theories that make the same verifiable predictions, the simpler theory is to be preferred. You will not find a single scientist that will disagree with that. Occam's razor cannot be proved or disproved, since it's not really within a formal system; but there are strong arguments "in favour" of it in information theory.
See the above statements by Cuch, please.
I rather believe I can find quite a lot of scientists who disagree with this.
The ones who trained me.
The ones that eg. understand that quantum mechanics is a nice piece of math that we're building on, but the accepted understanding of said math is little more than a game of charades because all we have is the math - if multiple theories explain the math and we have no way of checking them against eachother, then these theories are ultmately equally valid.
The ones that advice against using Occam's razor, because the probability that you have not fully analysed the situation and will thus mis-apply said razor is greater than the opposite.
Even if we choose to accept the applicability of the razor, we are not ready to wield it, because we do not know yet whch solution is the simplest.
Furthermore, we have no real data to compare to - the whole situation if fictional and the only spell we have which uses the guideline has been mentioned above. We cannot test the guideline, we can simply hope for a declaration of 'fiat' by the Line Editor.
Even if a supplement turns up stating one thing or the other, this will be a decission that cannot be assumed untanted by this discussion.
Tellus you were doing so well too...
They still use Occam's razor, neh?
They just take care about when to apply it.
Though I agree that the scientific method (build theory, test it, re-jig´theory, test, repeat ad nausam) is in applicable.
Ezz: they do have a point - odds that the author of the spirit binding text considered this specific context are slim to none.
And for every situation I can think of besides this, the difference is moot.
Personally I like Tellus' version. Gives more purpose to MuVi and you can't just re-jig every spell to be D: Ring.
But that's because I apply Occam's Razor to the outcome I guess 
And finally, a question:
To use D: Ring, everything targetted has to be inside the ring, neh?
Where is the spell?
Magus A casts a spell on grog B.
Which one of them have to be inside the Ring? Both of them?
Can you give canon spells that extend Duration using a "sustain" Guideline? Any of those for MuVi/ReVi?
Or is this "sustain" usage unrelated?